1. Field of Invention
This invention pertains to methods of producing foams utilizing a multifunctional isocyanate and a multifunctional polyol, and the foams made thereby. The examples pertain particularly to polyurethane, and polyurethane modified polyisocyanurate foams used for thermal insulation.
2. Related Art and Other Considerations
Cellular organic plastic foams made with urethane linkages, or made with a combination of both isocyanurate linkages and urethane linkages, are well known in the art. These foams have been made from the catalyzed reaction between polymeric polymethylene polyphenylisocyanate (Abbreviated "PMDI") and polyols of various physical and chemical properties. As used herein, the term "PMDI" defines any polymeric polymethylene polyphenylisocyanate which has an average functionality greater than 2.0. The PMDI has been used either alone, or in a blend with an expansion agent and (optionally) with a capped silicone, or other type of surfactant. Such a blend utilizing PMDI has traditionally been called the "A-Blend".
In order to form good cell size, good cell distribution, and cell-wall construction, it has sometimes been preferred to add other "plastic foam cell modifiers" to the foam formulations. It has been preferred to add these other agents to the polyol mixture, often called the "B-Blend". These foam cell modifiers include, but are not limited to: predominantly silicone surfactants, propylene carbonate, dispersing agents, organic surfactants, nucleating agents, fire retardants, expansion agent(s), and catalyst(s).
Organo-metal catalysts have been utilized in polyurethane reactions since implementation of the early stages of Otto Bayer's invention. It has been necessary to produce commercial products in the substantially anhydrous state, as most users could not tolerate water in their system. As defined for use herein, "substantially anyhdrous" means a mixture comprising less than five percent (5%) water by weight. The common organo-metal catalysts have utilized many different metal cations including, but not limited to: tin, antimony, lead, titanium, potassium, and sodium. The organo-portions of the molecules have been simple hydrocarbon groups such as: methyl-, ethyl-, propyl-, butyl-, and higher carbon chains such as from C.sub.8 up to C.sub.20. An example of an early and widely used urethane-chain reaction catalyst is dibutyl-tin-dilaurate. Other early catalysts utilized were the carboxylic acid salts of organo-metallic substances. For example, U.S. Pat. No. 4,246,356 teaches the use of stannous octoate in a flexible foam blown with CO.sub.2 from water and a CFC compound. This flexible urethane foam had no isocyanurate linkages due to the use of a two-functional isocyanate.
As polyurethane foams began to become modified with the three-ring trimer linkage made from three multi-functional isocyanates, other catalysts were investigated. For example, U.S. Pat. No. 3,940,517 to DeLeon incorporates prepared alkali metal carboxylate catalysts dissolved in glycols. DeLeon specifically mentions removing the water of reaction with a molecular sieve designated Linde 3A from Union Carbide Corporation. The resulting foam producing system is a substantially anhydrous chemical blend for making polyisocyanurate foam.
Another route utilized in prepared catalyst packages has been the use of quaternary ammonium salts in substantially anhydrous solvent systems. For example, U.S. Pat. No. 4,582,861 to Galla et al, teaches the use of both substantially anhydrous dibutyltin dilaurate catalysts and substantially anhydrous quaternary ammonium salt catalysts in foam systems which incorporated water to effect CO.sub.2 as a partial blowing agent. This disclosure incorporates U.S. Pat. No. 4,040,992, to Bechara et al, which discloses a condensation reaction, from which essentially all water is removed. A quaternary amine catalyst is not required in the scope of the present invention; however, one could be optionally added for enhancing the curing process.
Later, as the art of balancing expansion rates with chemical completion rates advanced, U.S. Pat. No. 4,710,521 to Soukup et al disclosed the advantages of premixing alkali metal organo-salt catalysts with certain select tertiary amine catalysts, in favorable ratios, all in a substantially anhydrous glycol solution. The resulting polyurethane or polyisocyanurate foams were all formed from substantially anhydrous chemical blends.
As a general rule, the prior art in closed-cell rigid foam insulation has been concerned with properly controlled reaction rates, whereby the expansion reaction would proceed quickly and smoothly ahead of the chemical polymerization reaction. These foam systems were all substantially anhydrous systems.
In fact, nearly all of the prior art references disclosing both urethane and trimerization reactions together have been substantially anhydrous systems. One exception has been found in U.S. Pat. No. 4,981,880 to Lehmann, which uses trimerization catalysts in the production of open-celled flexible foam. However, Lehmann does not use PMDI, nor does Lehmann use over 1.0 pphp (Parts Per Hundred Parts of Polyol) by weight of trimerization catalyst. The object of the low level of trimerization catalyst in Lehmann is to create some isocyanate dimerization linkages by utilizing the two functional Toluene Di-Isocyanate ("TDI"). This process may add strength to a flexible foam, but it does not create a rigid, solid, closed-cell foam.
It is considered an essential part of the present invention that the PMDI have an average functionality over 2.0. A rigid foam insulation must utilize the cross-linking ability of multifunctional PMDI with polyols. Such a reaction would not be used to produce open-celled flexible foam.
The growing popularity of the urethane modified polyisocyanurate foam insulation has created a demand for many forms of alkali metal organo-salt compounds, all of which have been substantially anhydrous systems prior to the instant invention. The most preferred prior art alkali metal organo-salt catalysts have been either potassium octoate, potassium acetate, sodium succinate, or other potassium or sodium cations with organic carboxylic acid anions, all of which have been in solution with ethylene-, propylene-, or diethylene- glycols, or in a combination of organic glycol solvents. While some commercial blends have contained small amounts of water (which the producers were unwilling or unable to remove), all such blends contain less than 5% water by weight.
Another class of prior art trimerization catalysts have been the quaternary ammonium salts in an anhydrous system utilizing glycol solvents. All of these prior art preferred catalyst systems have been substantially anhydrous systems. The manufacturers of these products have gone to great lengths to either vacuum distill, or molecular sieve filter, small amounts of water from these products so that the water does not exceed 5% by weight.
Since the beginning of CFC blowing agent phase-outs, the use of CO.sub.2 as part of the expansion agent system has been attempted with varying degrees of success. Novel methods of utilizing CO.sub.2 as a successful replacement for a minor portion of the required foam expansion in a rigid, closed-cell foam insulation, has been taught in U.S. patent application Ser. No 07/720,735 filed Jun. 25, 1991, now U.S. Pat. No. 5,252,625; and also U.S. patent application Ser. No. 07/851,889 filed Mar. 16, 1992, now U.S. Pat. No. 5,254,600; both of which are incorporated herein by reference.
The prior art also recognizes the need to maintain adequate cell-wall viscosity while the cells are rapidly enlarging. For example, U.S. patent application Ser. No. 07/495,616 filed Mar. 19, 1990, and U.S. patent application Ser. No. 07/720,735, filed Jun. 25, 1991, now U.S. Pat. No. 5,252,625 (all incorporated herein by reference) teaches the advantageous use of polyols with higher viscosities than prior art polyols. It shows that even using less HCFC-141b blowing agent than was previously used as CFC-11, the A-Blends and B-Blends could have unworkably low viscosities. If the liquid blends have viscosities too low, the cell-walls will rupture open prior to the chemical reaction firming them up. The use of polyols with higher viscosities is a novel approach to overcoming that particular problem.
The prior art alkali metal organo-salt compounds have been successful in the job of trimerizing three molecules of PMDI. However, the glycol solvents which have been used to "carry" the prior art alkali metal organo-salt compounds into the B-Blend are substantial users of isocyanate functional groups, to the detriment of the resultant foam. It is known to those skilled in the art, that a urethane foam made from ethylene glycol and PMDI will burn more easily than any other type of urethane foam. It is also too friable to be used to meet commercial foam insulation board standards.
In the commercially prepared potassium acetate materials, the major glycol carrier is ethylene glycol. When diethylene glycol (DEG) is used as the glycol carrier, a larger amount is required (as opposed to ethylene glycol) as a solvent in commercially prepared potassium octoate catalysts. Both ethylene glycol and DEG unfavorably react with the isocyanate functional groups. Hence, in order to maintain the same ratio of chemical equivalents (Index), the weight percent of PMDI must be increased in a foam utilizing a commercial catalyst containing glycols, as opposed to utilizing the catalyst of this invention.
Thus it is seen that prior art formulations utilizing commercial anhydrous alkali metal organo-salt compounds are more expensive due to higher levels of PMDI, and often times create a foam inferior in physical properties to the novel formulations presented in the instant invention.
It is therefore an object of the present invention to provide an improved method for the production of a rigid thermosetting plastic foam insulation.
An advantage of the present invention is that the formulations do not contain any unwanted hydroxyl functional groups carried by the alkali metal organo-salt catalyst compounds.
Another advantage of the present invention is the elimination of unwanted friability created by ethylene glycol urethane linkage.
Yet another advantage of the present invention is that in the event glycols are desired to modify the foam, complete control of which glycols to use is provided.
An advantage of the present invention is the creation of a more desirable relationship between the rate-of-expansion curve and the rate-of-reaction curve.
Another advantage of the present invention is the further reduction of cost provided by reducing the required percent by weight of PMDI to achieve any given chemical equivalents ratio.
Yet another advantage of the present invention is the creation of flame resistant amide groups, plus additional carbon dioxide amounts not obtainable in the prior art.
A further advantage of the present invention is the ability to produce foams, when desired, with low carbon dioxide content.
A further advantage of the present invention is the provision of a strong, economical, closed cell foam insulation which is characterized by a high degree of fire resistance, a high initial resistance to thermal conductivity, and a long-term thermal resistance. by weight.